What is ICD-10 code A48.1?

A48.1 is the ICD-10-CM diagnosis code for Legionnaires' disease. Legionnaires' disease is a serious lung infection caused by Legionella bacteria, typically acquired from contaminated water sources like air conditioning systems or hot tubs. It causes pneumonia with fever, cough, and breathing difficulties. A48.1 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. A48.1 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. A48.1 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. A48.1 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

Which HCC does A48.1 map to under V28?

A48.1 maps to CMS-HCC V28 HCC 282 (Aspiration and Specified Bacterial Pneumonias). The category numbers, labels, and conditions come from the CMS-HCC V28 model file for payment year 2026.

2026 ICD-10-CM A48.1: Legionnaires' disease

A48.1 maps to CMS-HCC V28 282. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · HCC coding software

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Code lookupA48.1

Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Certain infectious and parasitic diseases (A00-B99) / Other bacterial diseases (A30-A49)

A48.1

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Legionnaires' disease

Legionnaires' disease is a serious lung infection caused by Legionella bacteria, typically acquired from contaminated water sources like air conditioning systems or hot tubs. It causes pneumonia with fever, cough, and breathing difficulties.

Buddy Insight

Legionnaires' disease is a severe atypical pneumonia caused by Legionella pneumophila, typically acquired from contaminated water systems (cooling towers, hot water systems, decorative fountains).

Overview Risk Models Code Book Documentation Audit Notes Edition comparison

CMS-HCC V28

Mapped

HCC 282

Aspiration and Specified Bact...

Code-level coefficient reference

CMS-HCC V24

Historical

Not used for standard CY2026 Medicare Advantage payment. Select a historical period to review V24.

Not used for CY2026 payment

ACA/HHS

Mapped

HCC 163

HHS-HCC 163 adult, RAF varies...

Code-level coefficient reference

ESRD/PACE

Mapped

HCC 114

Aspiration and Specified Bact...

Code-level coefficient reference

RXHCC

N/A

—

Not risk-adjusted for this model/period.

Not mapped

Code Book Path

Official

A48Other bacterial diseases, not elsewhere classified A48.1Legionnaires' disease

Inclusion Terms

Official

No inclusion terms are included in this display for A48.1. Check the code and parent instructions in the Code Book.

Excludes 2

Official

carrier or suspected carrier of infectious disease (Z22.-)Inherited from A00-B99
infectious and parasitic diseases complicating pregnancy, childbirth and the puerperium (O98.-)Inherited from A00-B99
infectious and parasitic diseases specific to the perinatal period (P35-P39)Inherited from A00-B99
influenza and other acute respiratory infections (J00-J22)Inherited from A00-B99

Related Codes

Official

A48.0Gas gangrene A48.2Nonpneumonic Legionnaires' disease [Pontiac fever]A48.3Toxic shock syndrome A48.4Brazilian purpuric fever A48.5Other specified botulism

Includes

Official

diseases generally recognized as communicable or transmissibleInherited from A00-B99

Excludes 1

Official

certain localized infections - see body system-related chaptersInherited from A00-B99, A48
actinomycetoma (B47.1)Inherited from A00-B99, A48

Code First

Official

No Code First sequencing instructions are included in this display for A48.1. Check the code and parent instructions in the Code Book.

Use Additional

Official

code to identify resistance to antimicrobial drugs (Z16.-)Inherited from A00-B99

Code Also

Official

No Code Also instructions are included in this display for A48.1. Check the code and parent instructions in the Code Book.

Buddy Documentation Tip

HCC Buddy guidance

Legionella urinary antigen test positive (serogroup 1) or respiratory culture/PCR positive for Legionella

Chest imaging showing pneumonic infiltrates (often rapidly progressive, multilobar)

Clinical features supporting Legionnaires' disease (high fever, GI symptoms, hyponatremia, elevated LDH, rhabdomyolysis)

Epidemiologic exposure to potential water source (hospital, hotel, cruise ship, spa)

MEAT Support

HCC Buddy guidance

Legionella urinary antigen test positive (serogroup 1) or respiratory culture/PCR positive for Legionella

Chest imaging showing pneumonic infiltrates (often rapidly progressive, multilobar)

Clinical features supporting Legionnaires' disease (high fever, GI symptoms, hyponatremia, elevated LDH, rhabdomyolysis)

Epidemiologic exposure to potential water source (hospital, hotel, cruise ship, spa)

Audit Caution

HCC Buddy guidance

Legionella urinary antigen only detects serogroup 1 (the most common) — a negative test does not exclude Legionella; consider respiratory culture or PCR for other serogroups

A48.1 is a combination code — do not add a separate pneumonia code (J15-J18)

Legionnaires' disease is reportable to public health — ensure this is documented in the medical record

Nosocomial Legionnaires' disease should trigger infection control investigation and environmental testing of the hospital water system

Common Mistakes

HCC Buddy guidance

J15.9 (Unspecified bacterial pneumonia) — Use A48.1 when Legionella is identified; generic pneumonia codes miss the specific organism and treatment implications

A48.2 (Nonpneumonic Legionnaires' disease / Pontiac fever) — Pontiac fever is a self-limited flu-like illness without pneumonia; A48.1 is the pneumonic form

J15.0 (Pneumonia due to Klebsiella pneumoniae) — Both cause severe pneumonia, but Legionella has characteristic hyponatremia and does not respond to cephalosporins

Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →

Is A48.1 an HCC code?

Yes. A48.1 (Legionnaires' disease) maps to HCC 282, Aspiration and Specified Bacterial Pneumonias under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.440. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.

Coder answer: A48.1 is billable and maps to V28 HCC 282, Aspiration and Specified Bacterial Pneumonias. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code: A48.1
Description: Legionnaires' disease
HCC (V28): HCC 282 — Aspiration and Specified Bacterial Pneumonias
RAF reference coefficient: 0.440
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 282, Aspiration and Specified Bacterial Pneumonias

0.440

ESRDHCC 114, Aspiration and Specified Bacterial Pneumonias

Not separately weighted

These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. Weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.

Work A48.1 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT review for A48.1

For A48.1, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule.

MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
EEvaluate: test results, medication response, or physical findings reviewed by the provider
AAssess: explicit mention in the assessment or plan with acknowledgment of status
TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis

Coder workflow notes

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What This Code Means

A48.1 is the ICD-10-CM diagnosis code for legionnaires' disease. Legionnaires' disease is a serious lung infection caused by Legionella bacteria, typically acquired from contaminated water sources like air conditioning systems or hot tubs. It causes pneumonia with fever, cough, and breathing difficulties. A48.1 sits in the ICD-10-CM chapter for certain infectious and parasitic diseases (a00-b99), within the section covering other bacterial diseases (a30-a49).

Under the CMS-HCC V28 risk adjustment model, A48.1 maps to Aspiration and Specified Bacterial Pneumonias (HCC 282) with a source-labeled community, non-dual, aged reference coefficient of 0.440. No V24 mapping is shown for A48.1; use the applicable model and payment year when reviewing the V28 mapping. For CY2026 non-PACE Medicare Advantage, CMS uses 100% of the 2024 CMS-HCC model (V28). PACE uses a separate model blend. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.

Legionnaires' disease is a reportable condition in most jurisdictions; verify reporting requirements in your facility's compliance protocols. For A48.1, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.

HCC Buddy maintains structured V28 and V24 mapping, source-labeled coefficient references, and MEAT documentation criteria for A48.1 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Legionnaires' disease is a reportable condition in most jurisdictions; verify reporting requirements in your facility's compliance protocols
•Confirm the diagnosis is specifically Legionnaires' disease (A48.1) rather than Pontiac fever (A48.2), which is a milder form of Legionella infection

Clinical Significance

Legionnaires' disease is a severe atypical pneumonia caused by Legionella pneumophila, typically acquired from contaminated water systems (cooling towers, hot water systems, decorative fountains). It carries 5-10% mortality in community cases and up to 25-30% in nosocomial cases. It is clinically significant because standard beta-lactam antibiotics used for typical pneumonia are ineffective — Legionella requires macrolides or fluoroquinolones.

Documentation Requirements

✓Legionella urinary antigen test positive (serogroup 1) or respiratory culture/PCR positive for Legionella
✓Chest imaging showing pneumonic infiltrates (often rapidly progressive, multilobar)
✓Clinical features supporting Legionnaires' disease (high fever, GI symptoms, hyponatremia, elevated LDH, rhabdomyolysis)
✓Epidemiologic exposure to potential water source (hospital, hotel, cruise ship, spa)
✓Public health notification documentation as this is a nationally notifiable disease

Commonly Confused Codes

•J15.9 (Unspecified bacterial pneumonia): Use A48.1 when Legionella is identified; generic pneumonia codes miss the specific organism and treatment implications
•A48.2 (Nonpneumonic Legionnaires' disease / Pontiac fever): Pontiac fever is a self-limited flu-like illness without pneumonia; A48.1 is the pneumonic form
•J15.0 (Pneumonia due to Klebsiella pneumoniae): Both cause severe pneumonia, but Legionella has characteristic hyponatremia and does not respond to cephalosporins